Interpretive Summary: Spotted knapweed is a noxious weed that is found throughout the United States. This weed was introduced into the U.S. in the 1800’s and has since infested numerous rangelands, pastures, forests and roadsides. Knapweed is especially harmful to livestock and other grazing animals and can out-compete important native plants. Researchers have been studying ways to control this weed using safe, cost-effective and environmentally friendly methods. One effective means to control spotted knapweed is by using biocontrol agents, such as the knapweed root weevil, a member of the snout beetle family. This weevil lays its eggs in the roots of the plants which, once they hatch, eat the root and slowly kill the plant. However, low egg laying rates by the adult females and inconsistent developmental times of the younger weevils, as well as an inability to obtain adults year round, can limit their use in the fight against knapweed. One objective of this study was to develop a diet that would support, on a year-round basis, the consistent growth and development of the knapweed root weevil and would encourage dependable egg production. A second objective was to determine if the weevils could be stored in low temperatures for short periods of time, and afterwards be revived, to allow for coordinating insect supplies with their demand by researchers and weed control professionals. Our accomplishments included: 1) the development of insect diets that enabled us to grow this weevil in laboratories and/or greenhouses throughout the year; 2) the ability to store younger weevils at lower temperatures that, after completion of their development at normal temperatures, were able to mate and lay healthy eggs. The information gained from our accomplishments will assist researchers and insect control personnel in their ability to grow and/or stockpile knapweed root weevils. Having these weevils readily available on a year-round basis will allow research teams, biological control companies, state and federal insect control agencies, and many others, to expand their efforts to: 1) better understand how to effectively use the knapweed weevil in the fight against spotted knapweeds; 2) directly control knapweed infestations whenever and wherever needed.

Technical Abstract:
Cyphocleonus achates, the knapweed weevil, is an effective biological control agent of the invasive weed, Centaurea maculosa Lam. (knapweed). A meridic diet was developed and tested for the rearing of the larval stage of this insect. Using this diet, C. achates was reared for over three generations, with the adults being offered knapweed plants for feeding and oviposition in greenhouse conditions. Additionally, a comparison was made between our standard meridic diet formulation and one containing knapweed tissues, with slight or no differences being seen between the two. The following life history parameters were monitored over the three generations: percent egg hatch (ranging from 42.9 to 59.1%), time to egg hatch (ranging from 20.0 to 23.2 days), time to adult emergence (ranging from 52.0 to 54.1 days), adult weights three days post-eclosion (ranging from 101.9 to 117.0 mg), percent adult emergence (ranging from 48.3 to 58.6%), and percent mortality/deformity in the different stages (with mortality occurring primarily in the larval or pupal stages). An additional study involving low temperature and short day conditions suggested that C. achates could be maintained for longer periods of time in larval diet cells when placed in growth retarding conditions. Furthermore, external morphology was studied in order to distinguish between the sexes to ensure that each ovipositional cage had a similar ratio of females to males, with abdominal features being found to be the most dependable characteristics for this purpose.